1. Field of the Invention
This invention relates to the field of copolymeric compositions suitable for use in forming contact lenses. More particularly, the invention concerns a disposable, ultrathin ocular membrane formed from the aforementioned composition, which membranes can be worn for long periods of time without discomfort; the method of making such a membrane, and the membrane itself. p 2. Description of the Prior Art
Optical contact lenses are conventionally made of either hard plastic such as acrylic resins or materials which have been treated to make them hydrophilic and water swellable (the so-called "soft lens"). The resins utilized conventionally must, of course, possess the various properties required for the ophthalmic use with respect to their light transmission, durability, etc. However, all of the known lenses present the basic defect that they cannot be worn regularly in the eye for long periods of time, i.e. for more than about fourteen to eighteen hours. Most certainly, their use is not recommended during sleep. Consequently, the contact lenses known to date should be inserted and removed daily from the eye.
Furthermore, those "hard" lenses made from the polymethylmethacrylate resins possess excellent rigidity and durability for handling by the user. However, they are highly impermeable to oxygen which is necessary for the proper metabolic functioning of the eye. These resins are also hydrophobic and as a result lack surface wettability. This produces fluctuations in the degree of their light transmission and refractive effect which result in changes in the visual acuity of the wearer over relatively short periods of time.
The so-called "soft" lenses, which are water swellable and hydrophilic must be maintained under conditions of sterility. This results in various inconveniences to the user.
Moreover, the "soft lenses" which are made from water swellable, hydrophilic polymers do not have the durability of the "hard" acrylic resin materials and are much more difficult to handle. Further, they are water swellable and can absorb an amount of water which is equal to many times the original volume of the lens. Consequently, the dimensions of such lenses are constantly undergoing change, which, in turn, changes the refractive index, and the refractive or ophthalmic properties of the lens.
Also, when such lenses are manufactured in the dry, unswollen state, the ultimate dimensions of the lens can vary greatly because minor deviations from standards will be amplified many times when the lens fully swells. Therefore, the "soft" lenses are relatively difficult to manufacture to the exacting tolerances required.